The present invention relates generally to transmitters used in aircraft transponders and in particular to transmitters used in connection with a Traffic Alert and Collision Avoidance System, or TCAS.
Pilots primarily rely on xe2x80x9csee and avoidxe2x80x9d techniques to prevent mid air collisions between aircraft. Various avionics systems also aid the pilot and air traffic control in maintaining aircraft separation. These systems, generally known as surveillance systems, may include: transponders and/or collision avoidance systems that exchange aircraft data between aircraft and/or between aircraft and air traffic control.
Transponders are transmitters/receivers located aboard aircraft. The transponder transmits a 1090 MHz encoded message containing information about the aircraft in response to interrogation signals received from a ground based radar or other TCAS equipped aircraft. This radar, known as a xe2x80x9csecondary surveillancexe2x80x9d radar, or simply secondary radar, transmits an interrogation signal at 1030 MHz. The interrogation signal contains three pulses. The time interval between the first and third pulses determine what type of information is requested: eight microseconds for identification; and twenty one microseconds for altitude. If the aircraft transponder is a Mode A transponder, the transponder can reply only to the identification request. If the aircraft transponder is a Mode C transponder, the transponder can reply to both the identification and altitude requests.
The Mode A and Mode C systems are unable to relay additional information or messages between the ground based secondary radar and the interrogated aircraft, other than the identification and altitude information. The Mode Select, or Mode S transponder provides additional capabilities over those available in Mode A and Mode C. Standards for Mode S communications are contained in the Radio Technical Commission for Aeronautics (RTCA) document, xe2x80x9cMinimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ARTCBS/Mode S) Airborne Equipment,xe2x80x9d RTCA/D0-181A, issued January 1992 and incorporated herein by reference.
In operation, a unique 24 bit address code, or identity tag, is assigned to each aircraft. Once per second, the Mode S transponder spontaneously and pseudo-randomly transmits, or xe2x80x9csquitters,xe2x80x9d an unsolicited broadcast, including the specific address code unique to the aircraft carrying the transponder, via first one and then the other of its two omnidirectional antennas. Whenever the Mode S transponder is not broadcasting, it is monitoring, or xe2x80x9clisteningxe2x80x9d, for transmissions, including interrogation signals.
The unique 24-bit address code, or identity tag, assigned to each aircraft is the primary difference between the Mode S and Mode A/Mode C transponders. Interrogations are directed only to the particular aircraft using this unique address code and the replies are unambiguously identified. The unique address coded into each interrogation and reply also permits inclusion of data link messages to and/or from a particular aircraft.
The Traffic Alert and Collision Avoidance System (TCAS) is an airborne system that interacts with the Mode S and Mode A/C transponder system to alert the pilot of potential collision threats. The TCAS transmits interrogations which are received and replied to by other aircraft and used to determine the locations of other aircraft relative to own aircraft position. The TCAS system is described in RTCA document D0-185 entitled: xe2x80x9cMinimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipmentxe2x80x9d, issued Sep. 23, 1983, consolidated Sep. 6, 1990 and in DO-185A, xe2x80x9cMinimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System II (TCAS II) Airborne Equipmentxe2x80x9d, dated December 1997 both of which are incorporated herein by reference.
FIG. 1 illustrates one known embodiment of the TCAS system having 4-element interferometer antennae 2A and 2B coupled to a radio frequency receiver 3 of a TCAS processor 4. Receiver 3 is in turn coupled to a signal processor 5 operating known traffic alert and collision avoidance software. A radio frequency transmitter 6 is coupled to signal processor 5 for transmitting Mode S and Mode C interrogation signals. An associated control panel 7 for operating the TCAS system and a display 8 for displaying the TCAS information are each coupled to signal processor 5 of TCAS processor 4 as described in U.S. Pat. No. 4,855,748 and in co-pending U.S. application Ser. No. 09/369,752 entitled: xe2x80x9cMultifunction Aircraft Transponder,xe2x80x9d filed Aug. 6, 1999, each of which is incorporated herein by reference.
Transmitter 6 can consist of either a vacuum tube cavity oscillator or, more commonly, a solid state amplifier driven by an oscillator. The oscillator provides a radio frequency wave that is modulated by modulator 19. To satisfy reception requirements, the maximum output power of the transmitter is 1000 W. In typical designs, the TCAS must deliver nominally 250 W measured at the rear of the unit. FIG. 2 shows a typical Mode S transmitter and modulator in which an oscillator 20 drives an amplifier 21 composed of two zero bias Class C transistors 22 and 24. The difference between the output of amplifier 21 and the power delivered to the antenna accounts for subsequent losses at the transmitting antenna and intervening circuit components and ensures that sufficient output power is available at the transmitting antenna.
A ceramic resonator 30 preserves the frequency stability of the oscillator output. The output is additionally filtered to reduce or remove sideband harmonics which may be significantly amplified by amplifier 21 and subsequently received by other aircraft and confused with the true signal. The input signal from oscillator 20 is split into two signals separated by a phase angle of 90 degrees. Each output of power splitter 32 feeds one of transistors 22 and 24. The amplifier output is then supplied to a second power splitter 34 before being again filtered and supplied to one of antennas 2A and 2B.
One issue with use of the TCAS and secondary radar systems is that every Mode C/Mode A transponder that receives an interrogation signal replies. If the replies are received at the same time, or if portions of the reply signal overlap, the reply will be garbled. In addition, if an aircraft is interrogated by more than one ground facility or aircraft, a facility or aircraft other than the interrogator may receive the reply pulse. This unsolicited response is called FRUIT.
To minimize garble and FRUIT, the TCAS/transponder transmitter broadcasts the interrogation pulse using a technique called xe2x80x9cwhisper/shout.xe2x80x9d The xe2x80x9cwhisper/shoutxe2x80x9d technique uses a variable level attenuator to transmit the interrogation pulse at varying power levels. The sequence of variable power interrogations reduces the amount of garble that the receiving TCAS must process and reduces the amount of FRUIT added to the secondary radar system.
FIG. 3 contains a block diagram of the TCAS transmitter device showing amplifier 21 and a whisper/shout attenuator 40. After the signal to be transmitted is amplified by amplifier 21 it is then sent to attenuator 40 and phase shifters 42a-42c before being broadcast from antennas 2A and 2B. Because attenuator 40 and phase shifters 42a-42c occur downstream of amplifier 21, these components must be sized to handle the up to approximately 600 W of power output by amplifier 21. In addition, much of the output power generated by amplifier 21 is then wasted by attenuator 40 when in the whisper mode. In addition, because the Class C amplifier contained within amplifier 21 does not amplify linearly, the amplifier will introduce some amplification of the radio frequency pulse harmonics. This unwanted amplification is called spectral regrowth and an output filter is required to minimize its generation. The Class C amplifier can also only be turned fully on or fully off.
The present invention recognizes the problems inherent in the transmitter of the prior art. The present invention provides a TCAS and/or transponder transmitter that enables use of lower cost, smaller size components with fewer internal losses and which provides for more efficient amplification of the transmit signal.
According to one aspect of the present invention, the present invention includes a plurality of modulated radio frequency (RF) signal paths to be transmitted at the device antenna. A plurality of amplifiers are located in series along each of these signal paths. Each of the amplifiers is coupled to a control signal useful for activating the amplifier and/or for controlling the amount of amplification to be provided by that amplifier. In this manner, only the amount of amplification desired for that particular transmission need be provided. The present invention thus reduces the transmitter total power consumption as well as minimizing the cooling requirements of the device. Furthermore, the present invention thus provides the capability to deassert an entire transmission signal path. Such a capability is useful for isolating a faulty transmission path and/or to minimize signal leakage during times when a signal is being received. The distributed power channels therefore have the additional advantage that should a channel be faulty, or deasserted, the TCAS function can be performed by the remaining channels. Distributing the power across the plurality of signal lines also enables lower power and hence, smaller, less expensive components to be used on each of the signal paths.
According to another aspect of the present invention, the amplifiers arranged in series along each of the RF signal paths comprise linear Class A/B amplifiers. The linear amplification provided minimizes spectral regrowth and harmonics associated with the pulsed RF signal. The present invention thus eliminates the requirement for an output filter to remove these undesirable harmonics prior to transmission. The present invention thereby reduces the cost and complexity of the transmitter.
According to still another aspect of the present invention, the discrete amplification levels enable the transmitter to be constructed without the whisper/shout attenuator required in the prior art design. Optionally, a preferred embodiment of the present invention includes a programmable whisper/shout attenuator located upstream of the amplification circuit. The whisper/shout attenuator of the present invention thus operates at low power. The whisper/shout attenuator of the present invention can therefore be constructed as an integrated circuit or other surface mounted component and need not be sized for the high power loads found in the prior art designs. This feature also saves on the size, cost, weight and thermal efficiency of the transmitter. In addition, the whisper/shout attenuator of the present invention contributes to transmitter efficiency since the whisper/shout attenuator operates upstream of the signal amplifier and need not operate to dissipate the power generated during amplification of the signal as in the prior art device.
According to yet another aspect of the present invention, the present invention includes phase shifters on several of the RF signal paths upstream of the amplification. In a preferred embodiment, the phase shifters additionally comprise an IQ modulator, which in addition to controlling phase also permits control of signal amplitude. The design of the present invention also means that the phase shifters operate at low power levels and not at the high power levels found in the prior art devices. Thus, the phase shifters utilized in the transmitter of the present invention can include surface mounted integrated circuits, for example, of the type presently utilized in the cellular phone industry. The transmitter cost, complexity, size, weight and transmission path losses are thereby reduced over that found in the prior art.
Additional features and advantages of the present invention will be readily apparent from the detailed description and drawings as provided below.